The present invention relates to a control for a movable diffuser wall in a centrifugal compressor, and more particularly to calibrating the control when the position of the wall in the diffuser is unknown.
Various schemes have been utilized in the past to increase the efficiency of centrifugal compressors, such as using variable speed motors to vary the rotation of the impeller. Another scheme involves the use of vanes, both fixed and adjustable, in the diffuser section of the machine. Yet another scheme towards improving both the efficiency and operating range of a centrifugal compressor is through the use of a variable width vaned diffuser. In this case, the diffuser contains a movable wall that can be selectively positioned in regard to a fixed wall to control the flow of refrigerant therebetween. A centrifugal compressor employing this movable wall feature is disclosed in U.S. Pat. No. 4,527,949, which is assigned to the Assignee of this application. As disclosed in U.S. Pat. No. 4,527,949, the inlet guide vanes of the compressor are used in a conventional manner to regulate the mass flow of refrigerant through the machine while the diffuser wall position is varied to prevent surging.
In U.S. Pat. No. 4,503,684, a correlation has been incorporated into the compressor control to relate inlet guide vane positioning with diffuser wall positioning.
In the latter two cases above, a need has been discovered for calibrating the control apparatus for the diffuser wall when the location or position of the diffuser wall is unknown. This situation can exist whenever a loss of power to the control occurs, such as would occur when the system is turned on and off between seasons, during start up after maintenance, or when no earlier data exists in the control regarding the wall position due to intermittent power supply, or for other reasons.